As is well known in the art, bauxite is the primary raw material which is suitable for production of aluminum. Many of the bauxite deposits of a quality required for the Bayer process have almost become exhausted, with the result that extensive research has been conducted on other means of obtaining alumina compounds from other naturally occurring minerals, rocks and other substances which contain alumina.
An important technique for such recovery, which has received much recent attention, involves chlorination of these materials to produce anhydrous aluminum chloride. These chlorination processes can use, as raw materials, clay, bauxite of inferior quality, and other aluminum-containing materials. The essence of these processes is that some suitable reducing material is mixed with the starting aluminum-containing material and the mixture is treated with a suitable chlorinating agent. Coal, coke, or other carbon-containing material, e.g., carbon monoxide, are used as reducing agents. To obtain a product free from water, the physically and chemically bound water is first removed by preliminary heating and calcination.
It has been found, however, that the known processes of the type referred to above have several disadvantages. For example, the speed of the reaction is, from a practical point of view, too slow, while the conversion of alumina is considerably less than the theoretical value. This is probably due to the fact that, in the system to be chlorinated, the raw material containing the aluminum oxide forms a heterogeneous phase with the reducing material, and these two constituents are separated, even after thorough grinding and mixing, with resulting boundary surfaces which deleteriously influence the reactivity and speed of the reaction.
In accordance with the present invention, it has been found that the above disadvantages can be avoided, and excellent, nearly theoretical conversions to aluminum chloride can be obtained, if aluminum and the carbon-containing reducing agent are molecularly or substantially molecularly dispersed in a practically homogeneous phase, together with the chlorinating agent. Such conditions are satisfied by certain naturally occuring types of coal having a high ash-content. These include, in accordance with the coal-mineral classification, (1) coal slate, and (2) bituminous shale. Characteristically, the ash-content is from about 30 to 50% by weight for (1) and over 50% by weight for (2).
The ash of the above minerals are of two main types, depending on the geological circumstances of charring, these types of ash being the so-called clay and pyrite types. Particularly preferred are those raw materials which provide an ash with pyrite content. Furthermore, those materials providing an ash having a high aluminum content, i.e., those with at least 20% by weight of aluminum, calculated as Al.sub.2 O.sub.3, are preferred. Such material is to be found in the so-called knots rock, in which the carbon portion is in the form of very fine bituminite distribution, which is, from the point of view of chlorination, highly advantageous.
Thus the present invention comprises the process of production of aluminum chloride, free from water, by chlorination at high temperature. As raw material, coal slate, bituminous shale, carbonaceous and/or bituminous materials in which the ash content is at least 30% by weight and the ash contains at least 20% by weight of aluminum calculated as Al.sub.2 O.sub.3 are used. This starting material is preferably ground, and the ground phase is calcined with simultaneous coking. The material so prepared is subjected to the action of chlorinating materials such as chlorine gas at high temperatures, about 700.degree. to 1000.degree.C, preferably in the presence of other reducing materials, e.g., carbon monoxide, and the aluminum chloride product free of water, is separated from the high temperature gas mixture.